Thank you for the help, but the listing has unfortunately already ended (I didn't include a link, because active auction links are not allowed).
No worries though, because as a last resort the night before the auction was ending I looked into mail forwarding, and opened an account with a company who instantly gave me a UK address I could use. I got lucky and won, and the boots were forwarded to me super quickly (I won last Wednesday, they arrived at the forwarding company on Friday, and I got them yesterday - they could have delivered it already on Tuesday, but I wasn't able to pick it up that day), so my overall experience is positive
For small/cheaper items I don't think it would be worth doing this, but for a more expensive item like this, the postage they quoted was acceptable, so I'll keep this in mind for the future for similar items.
Mods, could you please close this topic down, as it is no longer relevant? Thank you! ^^